inotify: allow users to request not to recieve events on unlinked children

An inotify watch on a directory will send events for children even if those
children have been unlinked.  This patch add a new inotify flag IN_EXCL_UNLINK
which allows a watch to specificy they don't care about unlinked children.
This should fix performance problems seen by tasks which add a watch to
/tmp and then are overrun with events when other processes are reading and
writing to unlinked files they created in /tmp.

https://bugzilla.kernel.org/show_bug.cgi?id=16296

Requested-by: Matthias Clasen <mclasen@redhat.com>
Signed-off-by: Eric Paris <eparis@redhat.com>

fs/notify/inotify/inotify_fsnotify.c

@@ -22,6 +22,7 @@
  * General Public License for more details.
  */
 
+#include <linux/dcache.h> /* d_unlinked */
 #include <linux/fs.h> /* struct inode */
 #include <linux/fsnotify_backend.h>
 #include <linux/inotify.h>
@@ -157,6 +158,14 @@ static bool inotify_should_send_event(struct fsnotify_group *group, struct inode
 	mask = (mask & ~FS_EVENT_ON_CHILD);
 	send = (fsn_mark->mask & mask);
 
+	if (send && (fsn_mark->mask & FS_EXCL_UNLINK) &&
+	    (data_type == FSNOTIFY_EVENT_PATH)) {
+		struct path *path  = data;
+
+		if (d_unlinked(path->dentry))
+			send = false;
+	}
+
 	/* find took a reference */
 	fsnotify_put_mark(fsn_mark);